Forward difference quotient.
Shortens the signal by one.
Inverts Synthesizer.State.Filter.Recursive.Integration.run in the sense that
differentiate (zero : integrate x) == x.
The signal is shifted by a half time unit.

Central difference quotient.
Shortens the signal by two elements,
and shifts the signal by one element.
(Which can be fixed by prepending an appropriate value.)
For linear functions this will yield
essentially the same result as differentiate.
You obtain the result of differentiateCenter
if you smooth the one of differentiate
by averaging pairs of adjacent values.